If you fermented at 20C, you will have theoretically 0.83 volumes of Co2 disolved in your beer at end of ferment. You are looking for about 2.5 volumes so you must force in another 2.5- 0.8 = 1.7 volumes or 32 litres of gas in a 19L cornie. If you have 1 litre of head space pressurised to 200kpa...